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ULF/VLF: Tweak resonance measurements

To: [email protected]
Subject: ULF/VLF: Tweak resonance measurements
From: DK7FC <[email protected]>
Date: Sun, 09 Jun 2019 11:11:46 +0200
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Hi ULF/VLF friends,

Last wednesday there was a heavy local thunderstorm with cloud-earth but also with cloud-cloud lightnings. The latter produce well visible tweak resonance patterns in spectrograms taken from the H field. The fundamental frequency is at a wavelength where the distance between ground and D-layer is just Lambda/2, i.e. near 1650 Hz. But they also appear at integer multiples. Such peaks can reach in the order of 20 dB above the level besides the resonance frequency.
A reprocessed spectrogram from that night shows such tweaks for a time of about 6 hours.
They started in daylight, where the D layer refelction height is lower. Then the night set in and the resonance frequency went down to about 1638 Hz, which becomes visible in this spectrogram and plot:

Again it is a good result done by vlfrx-tools and SpectrumLab. SpecLabs 'long term average graph' (red colour in the spectrum) was set to different half time values, 10 minutes was one of the best values. The plot shows the frequency of the highest peak of that graph. Unfortunately the time stamps are not correct, but they are correct for the spectrogram.
It looks like the frequency is reasonably stable  from 21 UTC onwards.
Sometimes that frequency is jumping by a few Hz. I guess that is because the location of the lightnings is varying and this will have an effect. However the peak is not to small, so it could work to make use of that resonator for amateur transmissions?

Also the 2nd resonance at 3305 Hz is quite expressed. The antenna efficiency at that frequency is much higher so maybe it is a good idea to start in that range.

With some luck, there will be a time-stamped and streamable VLF station at DL0AO soon. Signals from DL0AO and DK7FC could be joined in vlfrx-tools and then analysed side by side in SpecLab. With the plotter, the resonance frequencys from both locations could be plotted synchronuously, which will give a better impression of the dependency of the peak resonance from the location.

It is the ideal time for analysing such resonances from lightnings, however not for transmitting amateur signals ;-)

73, Stefan

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